Many methods have been devised to generate alternating current by converting mechanical energy into electrical energy via electromagnetic principles. The basic principle requires the relative motion of an electrical conductor, usually a conductive winding, with respect to a magnetic circuit to induce the flow of an electrical current in the conductive winding. Most devices employ rotational motion of an electrical conductor with respect to a multiple magnetic circuit as a means to induce an alternating electrical current.
Alternators that employ a translating or reciprocating member are generally referred to as linear alternators. Linear alternators are often used with free-piston stirling engines to extract electrical power from the reciprocating elements. In these prior art linear alternators, permanent magnets are utilized to develop a magnetic flux field which flows through the magnetic circuit. This flux field is traversed by an electrical conductor, usually in the form of an inductive coil. There are some general advantages to this approach. Linear alternators equipped with permanent magnets are physically smaller, require less volume, and tend to weigh less than alternative approaches.
Free-piston engines of various types are known and have certain essential features common to them all. The variations of free-piston engines include, for example, a pair of opposed pistons in a single cylinder as described in U.S. Pat. No. 3,234,395; a central piston rod having end-pistons at opposite ends with a cooperating free-piston axially spaced from each end-piston, thus forming two pairs of free-pistons as described in U.S. Pat. Nos. 3,541,362; 3,501,087; and 3,347,215; opposed sets of pistons with each set attached to a common rod, the inner pistons of the sets cooperating in a single cylinder and the remote outer pistons of the sets in separate cylinders, as described in U.S. Pat. No. 4,480,599; and one pair of pistons on a single rod with separate cylinders for each piston as described in U.S. Pat. No. 4,532,431. The general principles of operation of these and related free-piston engines are well known, with combustion at appropriate times, often by Diesel cycle, providing the power strokes of the pistons combined with appropriate inlet and outlet valves and/or ports.
With respect to conventional engines, Caris and Nelson investigated the effect of compression ratio on the efficiency of production General Motors Corporation V-8 engines modified for high compression ratio and found that brake thermal efficiency was maximized at a compression ratio of 17:1. See, SAE Trans. 1959, 67, 112-124. They suggested that the increasing departure from theoretical behavior as the compression ratio is increased is due to a longer burn duration at these higher compression ratios and/or the dissociation of the combustion products resulting in a pressure limit. Edson later studied the dissociation issue and concluded that this was a second-order effect at practical compression ratios. See, SAE Progr. Technol., 1964, 7, 49-64. Thus it appears that Caris and Nelson effectively demonstrated that, in their engine geometry, burn duration is the major detriment to the efficiency increase expected with increasing compression ratio.
The spark ignition crankshaft engine is limited in compression ratio by autoignition of the end gases. Diesel engines can utilize high compression ratios but suffer from fuel injection rate limitations. Both types of engines are limited by the finite amount of time required for the fuel/air mixture to burn.
Another problem associated with conventional engines is the pollutants generated. Since the discovery of burning fuels for heat or energy, there has been a problem of emissions from such burning. These polluting emissions consist of unburned fuel as well as the by-products of combustion, such as carbon dioxide, soot, carbon monoxide, partially burned fuel, and oxides of nitrogen (NO.sub.x).
Currently, most hydrocarbon fueled trucks and automobiles in the United States are required to have a catalytic converter on the exhaust gas line to decrease the amounts of polluting emissions.
Catalytic converters may have either oxidizing or reducing catalysts. The oxidizing catalysts continue the oxidation of the partially oxidized compounds in the exhaust gas. Thus, carbon monoxide is converted to carbon dioxide; unburned hydrocarbons are converted to carbon dioxide and water. Reducing catalysts effect the reduction of the nitrogen oxides to nitrogen and oxygen. Some three way catalysts are known whereby all three pollutants are reduced to varying degrees. The drawback of three way catalysts is that their efficiency is a function of the air/fuel ratio and peak efficiency is at a very narrow range of air/fuel ratios. Furthermore, none of the pollutants are entirely removed, and extremely expensive catalysts, such as rhodium, may be required. See, "Internal Combustion Engine Fundamentals", John B. Heywood, McGraw-Hill, New York, 1988, page 655-6, and "History of the Internal Combustion Engine" J. R. Mondt, published by ASME in ICE vol. 8, (1989).
Another method proposed for the control of emissions from hydrocarbon fueled engines is by the use of oxygen enriched intake air. With 25% and 28% oxygen in the oxygen enriched air, there is a significant reduction of carbon monoxide and of hydrocarbons in the exhaust gas. But the nitrogen oxide emissions are increased substantially. The reference below also reports that the presence of increased nitrogen oxides was expected "because the oxygen enriched air increased flame speed and combustion progressed rapidly to produce extremely hot gases". See, "The Potential Benefits of Intake Air Oxygen Enrichment in Spark Ignition Engine Powered Vehicle", (SAE 932803) Ng, et al., (1993).
U.S. Pat. No. 5,117,800 describes a method for operating a diesel or spark ignition engine, with oxygen enriched air of up to 40% oxygen, preferably 24 to 28% oxygen. The method is taught to be especially important when using fuels that are difficult to oxidize. As to pollutants, the patent further teaches that there is a reduction in smoke but that "the NO.sub.x concentrations do increase with increasing oxygen addition."
NO.sub.x emissions originate primarily from oxidation of the nitrogen gas contained in air. The hotter the burning zone or the longer the combustion time, the more NO.sub.x is formed. Little if any NO.sub.x comes from the fuel since there are essentially no nitrogen compounds in hydrocarbon fuels. A typical analysis of automobile gasoline shows less than 0.1% nitrogen.
These pollution problems are not only present in the exhaust from moving vehicle engines such as those in cars or trucks, but also are present in the exhaust from large stationary engines. All of these engines produce large amounts of nitrogen oxides as well as carbon monoxide and partially burned hydrocarbons. To date, no satisfactory method has been found to completely control the emissions of any one of these three types of pollutants. Nitrogen oxides are the most difficult to even partially control and frequently are increased by some methods used to control the other pollutants. Yet nitrogen oxides are very detrimental and produce a brown haze that is noticeable over many inland valleys and prairies, and when combined with water vapor, produce nitric acid an acid rain component.